In order to determine the direction of arrival of an electromagnetic signal, it is desirable for the antenna used to capture the signal to be large with respect to the wavelength of the signal. For example, in the case of an HF signal, the size of the antenna should theoretically be as much as several hundred meters. Thus, if the direction of arrival of the signal is measured from a platform of limited size such as a ship or a naval base, the antenna which is used generally has a special geometry enabling its dimensions to be reduced. In most cases, the antenna includes a monopole and two crossed loops, this type of antenna being commonly known as a “Watson-Watt antenna”, owing to the eponymous algorithm which is conventionally used to determine the bearing angle of an incident signal.
However, when operating from either a land- or sea-based platform, if it is desired to determine the direction of arrival of an electromagnetic signal emitted by a remote transmitter placed at ground level, in other words if it is desired to determine the bearing angle of arrival of a signal having a zero or quasi-zero elevation angle, the measurements are sometimes biased by the detection of waves having non-zero elevation angles and non-vertical polarization. This is because, in some cases, some waves captured by the antenna are initially emitted from the ground but are then reflected by the ionosphere which modifies their polarization. Incorrect values will then be obtained for the bearings if the Watson-Watt algorithm is used.
Furthermore, measurements of the direction of arrival of a signal are sometimes disturbed by the presence, in the proximity of the antenna, of physical structures which create reflections and parasitic coupling. This is the case, for example, on a ship, whose structure and external elements modify the electromagnetic environment around the antenna.
Finally, the known techniques cannot be used to estimate the quality of a bearing angle measurement which is obtained. It is thus impossible to distinguish reliable measurements from erroneous measurements.